Until my father explained it to me later, I did not understand the subtlety of Tom’s predicament: he would not have dared strike a white woman under any circumstances and expect to live long, so he took the first opportunity to run – a sure sign of guilt.

– Harper Lee

To Kill a Mockingbird, Chapter 19. Scout realized that Tom Robinson had no choice, one option was as bad as the other, because being a black man weighed against him in the legal system. Had he defended himself against Mayella Ewell he would likely have been killed, but running away made him look guilty.